Seismic sensor array with electrical to optical transformers

ABSTRACT

A seismic sensor array includes a means for passive electrical to optical energy transformation and transmission. This transformation is used remotely with traditional sensor arrays, which may include hydrophones, geophones, or a combination of them. The transformation means is used to develop an optical signal in a fiber which then conveys the seismic signals to a recording center or data accumulator.

FIELD OF THE INVENTION

The present invention relates generally to the field of seismic sensorarrays and, more particularly, to a hydrophone or geophone sensor arraythat receives an acoustic signal, develops an electrical signalrepresentative of the acoustic signal, and, through a plurality ofelectrical-to-optical transformers, converts the electrical signal to anoptical signal for transmission to a central receiver.

BACKGROUND OF THE INVENTION

In seismic exploration, whether on land, at sea, or in a transitionzone, large numbers of sensors are coupled together in groups to receivean acoustic signal and send a signal to a central receiver fordetection. This signal is representative of geological structures whichare analyzed for the likelihood of bearing hydrocarbons.

The presentation of such geological structures with adequate clarity andresolution requires large quantities of data. This data must be acquiredover a long seismic cable or marine seismic streamer and conducted backto the central receiver. Current systems use optical fibers to carry thevast quantities of data due to the relatively large bandwidth of fiberoptics and their relative immunity to many kinds of electromagneticinterference.

One of the weak links in the system just described is the portion whichtakes the acoustic signal and develops a signal for transmission to thecentral receiver. Recent developments have focused on fiber-opticsensors, so that the acoustic signal itself is used as the modulationmeans, creating a time varying effect on the optical path length of anoptical fiber under the influence of the acoustic signal. These systemshave proved to be effective, but technical developments remain beforethey reach large scale manufacture. In the meantime, there remains aneed for some means to use the electrical signal from a conventionalsensor, such as a hydrophone or a geophone, and convert that electricalsignal into a useful optical signal for transmission to the centralreceiver and/or a recording medium.

SUMMARY OF THE INVENTION

The present invention addresses this need in the art by providing ameans for passive electrical to optical energy transformation andtransmission. This transformation is used remotely with traditionalsensor arrays, which may include hydrophones, geophones, or acombination of them. The transformation means is used to develop anoptical signal in a fiber which then conveys the seismic signals via asignal conditioner to a recording center or data accumulator.

In one aspect of the present invention, the system, including an opticalfiber, a source, and a receiver, passively and remotely interrogates aplurality of electrical sensor strings by the use of electrical to fiberoptical energy transformers, which are located at spatial nodes along afiber optic transmission bus. Each node may constitute one seismic datachannel.

The present invention is particularly adapted for use with networkarrays and architectures of traditional seismic sensors. Any of theknown configurations of sensor strings may be passively and remotelyinterrogated with this invention.

A feature the invention is the electrical to fiber optical transformer.This transformer is a device that converts electrical energy tomechanical energy that in turn modulates the amplitude and/or phase ofoptical energy traveling within an optical fiber wave guide bus cable.This cable may have only optical fibers, a strength member and aprotective outer jacketing with discretely placed nodal takeouts wherethe optical fiber is tapped into.

In a preferred embodiment, mechanical strain of a single mode fiberwaveguide modulates the phase of a propagating light wave within thefiber core and/or cladding at each node. In optical fiber technology,single mode fiber is optical fiber that is designed for the transmissionof a single ray or mode of light as a carrier and is used forlong-distance signal transmission. This modulation takes place withrespect to a reference leg or compensating section of a fiber opticinterferometer. Various forms of channel multiplexing on a given fiberbus may be employed. These multiplexing methods may include the domainsof time, wavelength, frequency, phase, coherence, polarization, etc. Inother embodiments, a multimode fiber may be employed.

In operation, a conventional electrical sensor array is deployed in thetraditional fashion with each seismic channel (typically a plurality orstring of such electrical phones) electrically connected to itsrespective transformation node along the fiber optic bus line. In thisembodiment, no remote electrical power source, data concentration boxes,analog-to-digital converters, heavy copper transmission cable, or otherremote data acquisition components are required. This approach providesfor totally electrically passive remote interrogation of the seismicsensor array via low-cost, lightweight, all-dielectric optical fibercable and other ancillary state-of-the-art optical narrow/broadbandsources, receivers, couplers, connectors, switches, filters, gratings,isolators, circulators, reflectors, and modulators.

These and other features of this invention will be apparent to thoseskilled in the art from a review of the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an overall schematic diagram of a system of the presentinvention which employs electrical to optical transformers in a seismicexploration system.

FIGS. 2a, 2 b, 2 c, and 2 d are schematic diagrams of preferredembodiments of an electrical to optical transformer.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

FIG. 1 is an overall schematic diagram of a seismic array 10 in whichthe present invention finds useful application. Such an array may beincluded in a land system, a marine system, or a transition zone system.The system includes a central receiver 12 of the conventional type toreceive and record seismic signals from a plurality of optical datatransmission fibers 14 a, 14 b, through 14 n. Any number of fibers maybe used, limited only by the capacity of the central receiver to processthe data streaming in. Each of the transmission fibers 14 a, 14 b,through 14 n is preferably a single mode fiber and is coupled to thecentral receiver 12 through an optical fiber connector 16 a, 16 b,through 16 n, respectively.

The optical fibers, which in operation carry the seismic data to thecentral receiver, are divided into sections, typically of about 400meters in length in a marine application, by optical fiber connectors18. On the transmission fiber 14 a, and between respective ones of theconnectors 18, are a set of nodes 1 a, 2 a, and so on for as manysections of fiber as are included in the system. Similarly, on the fiber14 b are a set of nodes 1 b, 2 b, and so on. At each node is anelectrical to optical transformer 20, described below with regard toFIGS. 2a, 2 b, 2 c, and 2 d. Either side of the transformer is a pair ofreflectors or reflector/filter pairs 22, which may be fiber optic Bragggratings, partial silver mirrors, or other appropriate means forchannelization and multiplexing of the light signal in the transmissionfiber.

Coupled at each of the nodes is a sensor string 24 which includes aplurality of sensors 26. The sensors may be geophones or hydrophones, orother phones which can detect a seismic signal as a time varyingpressure, position, velocity, acceleration, or other physical parameter.The sensors 26 develop an electrical signal that is representative ofthe seismic signal such as by piezoelectric elements, thus requiring noexternal energy source. This electrical signal is conducted through thestring 24 to a transformer, where the electrical signal creates a timevarying alteration in the optical path length within the datatransmission fibers.

FIGS. 2a, 2 b, 2 c, and 2 d depict schematic diagrams of electrical tooptical transformers of this invention. In FIG. 2a, a transmission fiber14 is held under tension by a piezoelectric driver 30. The driver 30 ispreferably formed as described in U.S. Pat. No. 5,632,841 and isavailable from FACE International Corporation, Norfolk, Virginia, underthe trade name THUNDER. When impressed with an electrical signal, thedriver 30 exhibits significant flexure, thereby altering the opticalpath length of a section 32 of the fiber between points 31 and 33 of thefiber joined to the driver. In this embodiment, the driver 30 is coupledto a set of electrical leads 34 and 36 which are the conductors of thesensor string 24. The driver 30 is included within the transformer 20 ateach node, and thus transforms the electrical signal from the sensors 26to an optical signal in the transmission fiber 14, such as fibers 14 a,14 b, through 14 n.

FIG. 2b depicts another embodiment of an electrical to opticaltransformer in accordance with this invention. In this case, a cylinder40 is formed of the piezoelectric material. The lead 34 is coupled to anexterior surface of the cylinder 40, and the lead 36 is coupled to andinterior surface of the cylinder 40, which develops a mechanicalresponse when impressed with a time varying electrical signal from theleads. The transmission fiber 14 is wound around the cylinder 40, andthe mechanical response from the time varying electrical signal thusvaries the optical path length of the transmission fiber. This modulatesthe light signal in the transmission fiber, which modulation is detectedin the central receiver.

The embodiment of FIG. 2c is similar in function to the embodiment ofFIG. 2b, except the leads 34 and 36 are coupled to opposite ends of astack 41 of piezoelectric disks 43. The transmission fiber 14 is heldlongitudinally through the center of the stack 41, potted or otherwisesecured in a longitudinal hole 45. Multiple longitudinal passes may bemade to increase sensitivity, three of which are illustrated in FIG. 2c.Impression of an electrical signal from the leads 34 and 36 causes axialchanges in length of the stack, which modulates the optical path lengthof the fiber 14.

FIG. 2d depicts yet another preferred embodiment of the transformer. Inthis case, the transmission fiber 14 is wound around a compliant mandrel42 which has a certain diameter in its rest state. The mandrel 42 iscoupled to an electrodynamic element 44, such as a geophone. Theelectrodynamic element 44 develops a longitudinal compression of thecompliant mandrel 42. This compression develops a change in the diameterof the mandrel 42, and this change in diameter is sensed as a change inthe optical path length of the fiber 14.

The common feature in each of the embodiments just described is the useof the electrical or mechanical signal from a conventional set of phonesto develop a mechanical response which in turn varies the optical pathlength of an optical fiber as a modulation signal. Other structures maybe developed building upon this common feature and such structures wouldfall within the scope of the appended claims.

The principles, preferred embodiments, and modes of operation of thepresent invention have been described in the foregoing specification.This invention is not to be construed as limited to the particular formsdisclosed, since these are regarded as illustrative rather thanrestrictive. Moreover, variations and changes may be made by thoseskilled in the art without departing from the spirit of the invention.

We claim:
 1. A seismic sensor array comprising: a. a sensor stringincluding a plurality of passive seismic sensors for developing ananalog electrical signal; b. an optical fiber; and c. a node couplingthe string to the fiber for applying the analog electrical signal to thefiber, the node comprising a transformer including a piezoelectricmaterial which varies in physical geometry in response to the analogelectrical signal, thereby varying the optical path length of the fiberin an analog fashion in response to a seismic signal detected by thesensors.
 2. The sensor array of claim 1, wherein the sensors comprisegeophones.
 3. The sensor array of claim 1, wherein the sensors comprisehydrophones.
 4. The sensor array of claim 1, wherein the node comprisesan electrical to optical transformer comprising an arcuate piezoelectricelement coupled to the optical fiber at two points of the fiber, whereinthe piezoelectric element is driven by an electrical signal from thesensors.
 5. The sensor array of claim 1, wherein the node comprises anelectrical to optical transformer comprising a cylindrical piezoelectricelement with the fiber wrapped around the piezoelectric element, whereinthe piezoelectric element is driven by an electrical signal from thesensors.
 6. The sensor array of claim 1, wherein the node comprises anelectrical to optical transformer comprising a stack of piezoelectricdisks with the fiber held longitudinally within the piezoelectric stack,wherein the piezoelectric stack is driven by an electrical signal fromthe sensors, and wherein the electrical signal causes the piezoelectricstack to expand and contract in the longitudinal direction, therebycausing the same change in dimension in the fiber held within the stackas the expansion and contraction of the stack.
 7. The sensor array ofclaim 6, wherein the fiber is directed through the piezoelectric stack aplurality of times.
 8. The sensor array of claim 1, wherein the nodecomprises an electrical to optical transformer comprising a. an axiallyoriented elongate compliant mandrel with the fiber wrapped around themandrel; and b. a piezoelectric driver coupled to the mandrel such thatmovement of the driver axially drives the mandrel, altering the diameterof the mandrel, wherein the driver is actuated by an electrical signalfrom the sensors.
 9. A seismic sensor array comprising: a. a pluralityof sections of optical fiber coupled together with optical couplers toform a transmission fiber with a predetermined optical path length; b.an electrical to optical transformer on each of the sections to receivean analog electrical signal, the transformer comprising a piezoelectricmaterial which varies in physical geometry in response to the analogelectrical signal, the transformers adapted to modulate the optical pathlength of the transmission fiber in an analog fashion in responsethereto; and c. a sensor string coupled to the transformer, the sensorstring comprising a plurality of passive acoustic sensors adapted todevelop the analog electrical signal.
 10. The sensor array of claim 9,wherein the sensor string includes a plurality of geophones.
 11. Thesensor array of claim 9, wherein the sensor string includes a pluralityof hydrophones.